Two main types of petroleum coke are produced as by products from oil bearing materials in the “so-called” upgrading process, namely, delayed coke and fluid/flexi coke. In view of only minor differences between fluid coke and flexi coke, they are often grouped together under the name of “fluid coke”. Due to the difference in the production technology employed, particularly in process temperature, a typical fluid coke has a lower volatiles content, a higher bulk density, higher sulphur (ca 8%) and ash content than a typical delayed coke. Physically, fluid coke comprises particles having a particle size of about 200 μm in diameter with an onion-like layered structure, while delayed coke is produced in the form of large lumps.
Worldwide production of delayed coke exceeds fluid coke by several times. In Western Canada, daily production of oil sands fluid coke exceeds 6000 tonnes. Mainly due to its high sulphur content, almost all fluid coke produced is being landfilled and added to the existing stockpiles of about 45 million tonnes. It has been suggested that the fluid/flexi coking is “a front runner” among technologies for upgrading heavy crude to transportation fuel (Furimsky, 2000). Moreover, it was predicted that the production of petroleum coke would increase as a result of the increased amount of lower-quality high-sulphur crude oils treated (Swain, 1997). An increase in sulphur in coke is also anticipated, as more sulphur has to be rejected to meet the increasingly strict regulations on sulphur in transportation fuel. Clearly, there is a need to develop new, preferably, beneficial uses for high-sulphur petroleum coke, particularly fluid coke.
As is well known in the field, fluid coke produced at high temperature is refractory, in that it has a graphitic or glassy surface and is considered unreactive. For example, this type of unreactive coke is produced when sulphur containing coal or oil is pyrolysed to produce volatile gases as fuels and residual refractory coke. The sulphur containing refractory coke has limited use in that although the coke can be combusted as a fuel, upon combustion, the sulphur is converted to waste gas-containing sulphur dioxide, which gas must be treated to prevent release of the sulphur dioxide to the environment.
Sulphur content is at the centre of the challenges to using petroleum coke and invariably determines the end market for the coke. Low sulphur coke (<2 wt %) is often used for the production of anodes and other high value products, while a coke with 2 to 5 wt % S is considered to be fuel grade. Although fluid coke constitutes a significant energy source having very high heating values (32-35 MJ/kg), its utilization as a solid fuel in conventional pulverized coal (PC) burners is limited and, more often, prevented by the heavy burden added on traditional lime/limestone-based flue gas desulphurization (FGD).
According to Anthony (1995), it was concluded that “fluidized bed combustion (FBC) is the best, and only available technology for burning alternative fuels” with elevated sulphur levels, such as petroleum coke. In a FBC boiler, sulphur and nitrogen are captured during combustion and become part of the ash produced. However, FBC is not a de-NOx and de-SOx technology for treating flue gas. Limestone is often added at a typical Ca/S ratio of 2 to capture sulphur. Despite the reported high efficiency, sulphur capture in FBC remains one of the key issues in improving economics of the technology. Other limitations identified with FBC include fireside fouling that is closely linked to high sulphur in fuel (Anthony and Jia, 2000). Further, ash production and disposal problems are related to sulphur content. Desulphurization prior to utilization has also been studied. Unlike coal, sulphur in coke is largely organic in nature. Mechanisms of desulphurization, therefore, involve the cleavage of C—S bonds. In 1970's, Tollefson's group in Calgary pioneered the desulphurization of coke using hydrogen, and later improved the efficiency with ground coke particles and NaOH. It was found that fluid coke was more resistant to desulphurization than delayed coke. Molten caustic leaching, which was originally developed for removing organic sulphur in coal, was applied to both fluid and delayed coke at 200 to 400 C, resulting in less than 1% of sulphur (Ityokumbul, 1994). It was found, however, that no process developed so far had proven to be economically viable. Recently, Furimsky (1999) suggested that gasification could emerge as another alternative for utilizing petroleum residues, including coke. A group at Tohuku University investigated gasification with various metal hydroxide catalysts (Yamauchi et al., 1999). In two separate studies carried out in England and Spain, petroleum coke was added to a typical industrial coal blend used in the production of metallurgical coke (Barriocanal et al., 1995; Alvarez et al., 1998). At the University of Alabama, petroleum coke was tested for remediating Sucamoochee soil (15 wt % oil) via a two-step agglomeration process (Prasad et al., 1999). Under the optimal condition, the remaining oil content in the soil was found to be below 200 ppm.
It is known that SO2 can be converted into elemental sulphur with reducing agents and a number of processes have been proposed for this purpose. As a group, they are termed “SP-FGD” (sulphur-producing flue gas desulphurization). Some examples are the coal-based Foster Wheeler process (U.S. Pat. No. 4,147,762), the BaS/SO4-based cyclic process (M.Olper and M.Maccagni “Removal of SO2 from Flue Gas and Recovery of Elemental Sulfur” Euro. Pat. Appl. No. 728,698 28 Aug. 1996), the Claus reaction-based McMaster-INCO process (U.S. Pat. No. 6,030,592) and the Na2S(aq)-based low-temperature process (Siu and Jia, 1999; Siu, 1999). To reduce SO2 to sulphur, CH4, H2 and CO gases are also used, often with a catalyst. A recently example is the CO-based process developed at MIT, in which a cerium oxide-containing catalyst is used (U.S. Pat. No. 5,242,673).